1. Field of the Invention
The present invention relates to fiber optic gyroscopes. More particularly, this invention pertains to a method and apparatus for reducing the magnitude of Shupe bias by manipulation of fiber leads to correct stress imbalances within the coil pack.
2. Description of the Prior Art
FIG. 1 is a schematic view of a fiber optic gyroscope. It generally comprises a light source 10, a beamsplitter 12 (either a fiber optic directional coupler or an integrated-optics Y-junction), a fiber optic coil 14, a polarizer 16 (and sometimes one or more depolarizers) and a detector 18. A beamsplitter 11 between the light source 10 and the polarizer 16 insures that the configuration is reciprocal. Light from the light source 10 is split by the beamsplitter 12 into copropagating and counterpropagating waves travelling in the sensing coil 14. The associated electronics measures the phase relationship between the two interfering, counter-propagating beams of light that emerge from opposite ends of the coil 14. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias or error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. These factors are both time-varying and unevenly distributed throughout the coil, inducing variations in the optical light path that each counterpropagating wave encounters as it travels through the coil. The phase shifts induced upon the two waves are unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
U.S. Pat. No. 5,329,349 of Ralph A. Patterson, et al. covering "Method for Tuning Fiber Optic Sensor Coils" ('341 Patent, property of the Assignee herein) teaches a method for addressing the bias errors that often persist as a result of the inevitable imperfections in applying well-known cancellation coil winding techniques such as quadrupole winding. In practice, bumps, wrinkles and various errors will invariably occur during coil winding. As a result of such errors, fiber segments equidistant from the coil midpoint may not be located in physical proximity (a requirement for an effective quadrupole winding). Winding errors are to be expected in view of the quite substantial task involved in the precise winding of a coil of, for example, one thousand meters of optical fiber. The above-referenced patent teaches that the results of such unavoidable winding imperfections, including time-dependent changes in fiber properties and bias effects can be addressed through the selective trimming of the end leads of the wound coil in response to observations (by means of test apparatus) of the coil's performance. According to theory, a Shupe bias point may be determined so that the Shupe bias error can be attributed to a difference in fiber lengths between that Shupe bias point and the ("CW" or clockwise and "CCW" or counterclockwise) ends of the coil. In theory, the trimming of the '349 Patent compensates for the fact that unequal lengths of optical fiber may exist between the Shupe bias point and the coil ends due to the above-described imperfections. By adjusting the lengths of the leads relative to one another, one effectively makes up or compensates for this asymmetry.
While the above lead trimming method will reduce bias errors, it is limited in effectiveness as it assumes that the Shupe bias error is the result of winding asymmetries alone and ignores other issues relating to the coil leads. The leads must be secured against the coil pack. Otherwise, very significant vibration-induced Shupe errors would be introduced. Unfortunately, the necessary anchoring of the coil leads introduces the factor of fiber stressing into the analysis of temperature-induced Shupe bias. Thus, this issue must necessarily be addressed to avoid the coil leads acting as a source of bias rather than a solution to such problem.